In recent years, a CIS-based thin film solar cell which uses a Group I-III-VI2 compound semiconductor of a chalcopyrite structure which contains Cu, In, Ga, Se, and S as a p-type light absorption layer has come into focus. This type of solar cell is comparatively cheap in manufacturing cost and further has a large coefficient of absorption in the wavelength range from visible to near infrared, so promises a high photovoltaic conversion efficiency. It is deemed as a promising candidate of next generation type solar cells. As typical materials, there are Cu(In, Ga)Se2, Cu(In, Ga) (Se, S)2, CuInS2, etc.
A CIS-based thin film solar cell is obtained by forming a metal backside electrode layer on a substrate, forming a Group I-III-VI2 compound, that is, a p-type light absorption layer, on that, and further forming an n-type high resistance buffer layer and n-type transparent conductive film forming a window layer successively on it.
The p-type light absorption layer is formed by forming a metal precursor film on the backside electrode by the sputtering method etc. and heat treating this by the selenization/sulfurization method. When using a metal precursor film constituted by a CuGa/In multilayer film, the p-type light absorption layer becomes a Cu(InGa)(SeS)2 layer. It is known that in this selenization step, Se reacts with the CuGa/In multilayer film (Cu+Se→Cu2Se, In+Se→In2Se3, Ga+Se→Ga2Se3) and that in the process, volume expansion occurs, strain occurs inside, and voids of several μm size are formed in the layer (for example, PLT 1, paragraph s (0005), (0006)).
It had been believed that such voids had a detrimental effect on the solar cell characteristics and were a cause of reduced photovoltaic conversion efficiency. Therefore, in the past, a method of production which prevented the formation of such voids as much as possible was employed.
On the other hand, in a crystalline Si solar cell which is made from a Si wafer, to further improve the photovoltaic conversion efficiency, it has been proposed to make the electrodes point contact structures. This has actually been applied. The contact interface between the semiconductor layer and the electrodes is a part which is high in dangling bond and other crystal defect density and where the carrier recombination rate becomes the highest. Therefore, in the prior art, the semiconductor layer and the electrodes are made to contact each other by points to reduce the ratio of front surface recombination and to improve the photovoltaic conversion efficiency. At the major part between the semiconductor layer and the electrodes, a good quality oxide film with a small speed of front surface recombination which functions as a passivation film is formed to realize a point contact structure and reduce the carrier recombination rate (for example, see PLT 2). Due to this, it is known that the open circuit voltage, one feature of a solar cell, is improved.
However, in a CIS-based thin film solar cell, the above-mentioned point contact structure is not realized. When applying the art for the above crystalline Si solar cell to a CIS-based thin film solar cell, it is necessary to form an insulating film between the semiconductor layer and the electrodes, but the art of forming such an insulating film has not been realized yet. While there is a possibility of realization by future technical innovation, in this case, it is believed that the production process becomes complicated and the production cost increases.
Furthermore, in a conventional Si-based solar cell, in particular a thin film Si solar cell, the light absorption coefficient of Si is low, so use of a BSR (back surface reflection) structure is the general practice. With this structure, light which strikes the front surface of the solar cell and passes through the semiconductor layer is again reflected at the back surface back to the inside of the semiconductor layer, so the incident light can be efficiently absorbed. The back surface internal reflectance is determined by the refractive index (N) of the semiconductor and the backside electrode and the angle of incidence of light to the back surface.
In this regard, in a CIS-based thin film solar cell, as the material of the backside electrode, Mo, Ti, Cr, etc. which are superior in selenium corrosion resistance are used, but these metals react with Se or S at the time of formation of the p-type light absorption layer. For example, when using Mo, MoSe2, Mo(SSe)2, or other reaction layer which has a refractive index of the same extent as a CIS layer is formed on the front surface of the backside electrode layer. For this reason, the reflectance between the two is low and an effective BSR structure is not realized.
As explained above, in a conventional CIS-based thin film solar cell, various measures have been taken to improve the photovoltaic conversion efficiency, but a point contact structure and BSR structure which are generally employed in solar cells made of Si and considered effective in improving the photovoltaic conversion efficiency have not been realized.    PLT 1: Japanese Patent Publication No. 2000-87234 A1    PLT 2: Japanese Patent Publication No. 9-283779 A1